1. Field of the Invention:
The invention relates to the removal of condensible vapors in gases.
2. Discussion of the Background:
In many applications of gases, such as the manufacture of VLSI chips, high purity gases are needed during the manufacturing process to avoid, among others, the creation of defects during the various masking steps of the process. Avoiding such defects enhances the manufacture output of the process as well as the reliability of the chips. There is also a need for high purity gases in the optical, aerospace and pharmaceutical industries.
Various impurities such as particles or vapors may be present in compressed gases stored in a container such as a cylinder. It is known from the article entitled "A gas filtration system for concentrations of 10.sup.5 particles/cm.sup.3 " from G. Kasper and H.Y. Wen; published in Aerosol Science and Technology 5: 167-185 (1986), that "totally" particle-free process gases may be obtained. Particle analysis has been also carefully studied by the same authors and the results of this study has been published in the Proceedings--Institute of Environmental Sciences--May 6, 1987--in an article entitled "Particle Analysis in cylinder gases".
Trace quantities of condensible vapors such as hydrocarbons or fluorinated hydrocarbons in gases are commonly detected and quantified by a variety of means such as chromatography in the gas phase, infrared (IR) absorption spectroscopy, mass spectrometry, total hydrocarbon detectors, etc.
Pressure reduction by critical orifice is commonly used in pressure regulators. For the purpose of sampling particles from compressed gases of pressure up to 500 psi, a pressure reducer, consisting of a clean orifice, a chamber into which the gas can expand, and a means to sample part of the expanding jet, can achieve "zero" particle shedding. Such a unit can be purchased from Liquid Air Corporation.
It is known that very high concentration of fine particles can be generated during the pressure reduction process if the upstream pressure exceeds a few hundred psi depending on the gas purity (J. Aerosol Sci., 19, 153, 1988). The particle concentration decreases with decreasing upstream pressure and drops to zero typically at pressures within the range of 300 to 600 psi. It is documented that these particles are formed from condensation of trace amount of vapor impurities in the compressed gases.
The removal of vapor impurities (purification) is conventionally accomplished with the help of molecular sieve or activated carbon. This approach however which may totally distort the original particle spectra and concentrations in the compressed gases.
Utilization of the differences in diffusion coefficients to remove unwanted nitric acid vapor from a gas stream to obtain true particulate nitrate is a common practice among air pollution researchers, where it is called "acid denuder". A basic coating is usually applied to the collection surfaces to retain the acid vapors, which may shed significant particles to upset the purpose of sampling particles from high-purity gases. The system is at ambient pressure. No temperature control is provided.
In view of the considerable demand for high purity gases and high purity gases containing particles there is therefore a strongly felt need for a method for separating condensible vapors from particles in a highly compressed gas. Such a method would ideally not affect the spectra or concentration of the particles in the compressed gas.
It is also known from U.S. Patent Applications Ser. Nos. 107,173 now U.S. Pat. No. 4,878,570 and U.S. Patent Application Ser. No. 107,177, both filed Oct. 13, 1987, and incorporated herein by reference, to generate droplets of condensible vapors in a carrier gas through a critical orifice and to discharge a carrier gas containing condensible vapors through a plurality of orifices without causing the condensation of the condensible vapors.